Mirror: Spelling and phrasing mistakes in skills markdown

[hamen]

This mirrors Issue Tracker bug https://issuetracker.google.com/issues/515090590.

The original report flags spelling, capitalization, spacing, phrasing, heading, numbering, and JSON example issues across several Android skill Markdown files.

A proposed fix is available as a pull request against my fork because pull requests are currently restricted on this repository:

hamen#1

The fork PR fixes:

• Trade-offs wording in the Compose styles skill.
• JaCoCo capitalization and heading levels in the testing setup skill.
• Missing sentence spacing in the Perfetto SQL skill.
• R8 build-time performance phrasing.
• Stale Antigravity-specific wording in the Android CLI skill.
• Duplicate numbering in the Android interaction rules.
• Malformed JSON keys and missing quote in the journeys example.

Validation included in the branch:

• bash scripts/check_issue_515090590.sh
• git diff --check

cc @JoseAlcerreca